Chemical methods for removing sulfur from coal currently being developed include oxidation (e.g., using oxygen, air, ferric sulfate, chlorine, nitrogen oxides, nitric acid, ozone, or hydrogen peroxide); caustic leaching; and reduction with hydrogen, zerovalent metals, and single electron transfer agents. Other methods include supercritical extraction with ethanol, and combination methods such as a three-step sequence of charting, acid leaching, and hydrodesulfurization. Microbial and "super-base" desulfurization is also being studied as is deoxygenation of phenols by CO via metal complexes. Sonication has also been employed to enhance oxidative desulfurization. For coal liquids, hydrodesulfurization (using hydrogen with or without supported molybdenum and cobalt catalysts) can remove approximately 90% of the organic sulfur under elevated pressure at temperatures up to 500.degree. C. None of these methods is yet completely satisfactory, however, especially for organically bound sulfur.
Coordination complexes such as Ir(S.sub.2)(dppe).sub.2.sup.+ and Cp.sub.4 Fe.sub.4 S.sub.6 are known to lose all or part of their sulfur to phosphines (PR.sub.3) to form phosphine sulfides (R.sub.3 P=S). See, A. P. Ginsberg et al., Inorg. Chem, 21, 3666 (1982) and G. J. Kubas et al., Inorg. Chem., 20, 2667 (1981). More, recently it was shown that PR.sub.3 compounds can reduce FeS.sub.2 to Fe.sub.0.95 S upon heating and that iron can remove sulfur from (n-Bu).sub.3 P=S at 300.degree. C. to give FeS. The FeS2 in Springfield No. 5 coal also was converted to FeS in the presence of (n-Bu)3P and Nujol at 260.degree. C. See, J. Hoots, Ph.D. Thesis, University of Illinois (1984), pages 133-183. However, this was not particularly efficient as only part of the sulfur was removed.
A severe problem with sulfur-bearing coals is that much, if not most, of the organic sulfur is present as thiophenes, which are particularly resistant to chemical desulfurization. See, L. M. Stock et al., Energy and Fuels, 3, 651 (1989) and W. H. Calkins, Fuel, 73,475 (1994). A specific example of such a thiophene is dibenzothiophene (DBT). ##STR1##
Thus, what is needed is a method for the effective and efficient removal of sulfur from coal or other sulfur-containing petroleum products, particularly those materials containing significant amounts of organically bound sulfur.